Touch probe with transmission device and spring for urging the device against guide surfaces

ABSTRACT

A touch probe with an arm ( 13 ) coupled to a movable arm-set ( 3 ) supported in a casing ( 1 ) includes an electric switch ( 31 ) with a housing ( 33 ) fixed to the casing and a mechanical transmission device ( 61 ) for mechanically transmitting displacements of the arm, and causing the disengagement of a conductive small ball ( 51 ) from two stationary conductive bars ( 44, 45 ), thereby opening an electric detecting circuit. An elongate mechanical body ( 63 ) of the mechanical transmission device includes a spherical-shaped element ( 67 ) that is transversally urged against a binary ( 71 ) for guiding with accuracy and free of clearances the displacements of the elongate mechanical body. A bent flat spring ( 73 ) contacts a plane portion of the spherical transmission element for achieving both the transversal thrust against the binary and the antirotation of the elongate mechanical body.

TECHNICAL FIELD

The invention relates to a touch probe, including a casing that definesa longitudinal geometric axis, a movable arm-set housed in the casing,that defines a longitudinal symmetry axis, an arm rigidly coupled to themovable arm-set, with an end extending out of the casing, a feelercoupled to said end of the arm, and an electric switch adapted fordetecting displacements of the movable arm-set with respect to thecasing and including a housing, at least a stationary contact, a movablecontact, and a mechanical transmission device adapted for transmittingdisplacements of the movable arm-set to the movable contact, themechanical transmission device including an elongate mechanical body,located between the movable arm-set and the movable contact, arrangedand movable substantially along a longitudinal direction, and guideelements for cooperating with the elongate mechanical body.

BACKGROUND ART

Touch probes with movable arm-sets that carry feelers are utilized incoordinate measuring machines and in machine tools, more specifically inmachining centers and lathes, for carrying out checkings on machined orto be machined pieces, on the tools, on the machine tables, etc. In eachof these probes, contact between feeler and, for example, a mechanicalpiece is monitored by suitable devices which detect specificdisplacements of the movable arm-set with respect to a casing andcontrol the reading of transducers associated with the machine slides,that provide measurement values with respect to a reference position ororigin.

A probe detecting and monitoring device can foresee the utilization ofan electric circuit and of (at least) an associated switch that,actuated in a mechanical way as a result of displacements occurringbetween the movable arm and the casing, causes the closure or, morefrequently, the opening of the circuit.

The U.S. Pat. No. 5,299,360 discloses probes according to the preambleof claim 1, each including a microswitch with a stem having a free endthat cooperates with the movable arm-set. More specifically, in each ofsaid probes, the coupling between the movable arm-set and the stationarycasing is such that, further to displacements of the feeler in alongitudinal direction or in a transversal direction, there occurs thelifting of an abutment surface of the movable arm-set that is near themicroswitch and the consequent thrust on the stem, the disengagement ofthe contacts and the opening of the electric circuit.

An extremely important characteristic that touch probes are required tohave is a high standard of repeatability i.e. a close correlationbetween specific positions taken by the feeler and the opening of theelectric circuit.

In order to improve said characteristic, the mutual arrangement amongthe component parts of the switch is accurately defined insofar as,among other things, the aligning—along a longitudinal axis of the thrustspring—of the movable contact, generally ball shaped, and of the stem isconcerned. Although the probes disclosed in U.S. Pat. No. 5,299,360guarantee good repeatability, an absolutely perfect alignment betweenthe component parts of the switch is not however possible. Furthermore,variations in the mutual positions of the stem and of the othercomponent parts of the microswitch due to clearances in the longitudinalguide system and/or possible rotations of the stem about its axis, eventhough of extremely small entity, can negatively affect therepeatability of the probe. This is especially true in recent highprecision applications in which it is required that the repeatabilityerrors be particularly small, considerably smaller than 1 μm.

Other inconveniences that occur in the known probes as, for example,those disclosed in the formerly mentioned U.S. Pat. No. 5,299,360, butnot just in those probes, hang on the reliability of the electriccontacts of the microswitch. In fact, although the probes are protectedby gaskets of various types, they cannot be, in general, consideredhermetically sealed, more particularly the rubber gaskets are notentirely sealed against oxygen and water vapour. These two elements,oxygen and water vapour, put together or separately, concur to oxidationprocesses of the microswitch electric contacts and hence lead to causemalfunctionings, that can affect the reliability of the probe. In orderto avoid the occurring of these negative processes, it is known toutilize, at the interior of the microswitch, lubricating fluid (“oil”)for protecting the contacts. However, although the presence of oilconsiderably improves the reliability of the contacts, it can interferewith the metrological performance of the probe, in particular negativelyaffect its repeatability. In fact, the instant when the opening of thecontacts—that are fed with very low voltages—is detected can be alteredin an unforeseeable way owing to the presence of oil on the contacts.

DISCLOSURE OF INVENTION

An object of the present invention is to provide touch probes that, bymaintaining the known structure substantially unaltered and by utilizingextremely simple and economic means, attain an improvement in theperformance and, in particular, an extremely high standard ofrepeatability.

This object is achieved by a probe according to claim 1. An importantadvantage that the invention provides is an effective improvement in theperformance of the probes, and thus a more accurate machining of thepieces controlled by said probes, by utilizing extremely simple andeconomic means.

BRIEF DESCRIPTION OF THE DRAWINGS

Other profitable features of the invention will appear from thefollowing detailed description when considered in connection with theenclosed sheets of drawings, given by way of non-limiting example,wherein:

FIG. 1 is a longitudinal cross-sectional view of a touch probe accordingto a preferred embodiment of the invention, with some details shown infront view;

FIG. 2 is an enlarged longitudinal cross-sectional view of a componentof the probe shown in FIG. 1; and

FIG. 3 shows the component of FIG. 2, when viewed from the bottom and inthe direction indicated by arrow III of FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The probe shown in FIG. 1 includes support means with a casing 1—that ishollow, of a substantially cylindrical shape and consists, for example,of a plurality of mutually assembled portions—that defines alongitudinal geometric axis, and a movable arm-set 3 housed andsupported in casing 1. Casing 1 includes two reference areas for themovable arm-set, more specifically a substantially frusto-conical seat 5and an abutment area 7 with plane annular surface. In turn the movablearm-set 3 defines a longitudinal axis of symmetry and includes a centralreference portion 9, having substantially the shape of a sphericalsegment defining a spherical zone, and an abutment flange 11 withannular surface. An arm 13 is coupled to the movable arm-set 3 and afeeler 15 is fixed to a free end of arm 13.

A helical spring 17 has its ends in abutment against two plane surfaces,integral with the movable arm-set 3 and the casing 1, respectively, andurges the central reference portion 9 against seat 5.

When movable arm-set 3 is in the inoperative condition of FIG. 1,symmetrically arranged with respect to the longitudinal geometric axisdefined by casing 1, the substantially spherical zone of the centralportion 9 is housed in seat 5 with substantially circular mutualcontact, whereas the annular surface of the abutment flange 11 is at adistance of a few micrometers from the abutment area 7 of casing 1. Theexistence of this clearance, that cannot be identified in the figure,determines the correct operation of the head according to the describedembodiment, as detailedly outlined in the formerly mentioned U.S. Pat.No. 5,299,360, to which reference can be made for a more detaileddescription.

It should be noted that a different embodiment of the presentinvention—that will be more extensively disclosed in the followingdescription—foresees, instead, that the abutment flange 11 be—when theprobe is in the inoperative condition—in contact with the abutment area7 of casing 1, urged by the thrust of spring 17. In this case there willbe a radial clearance of a few micrometers between the central portion 9of the movable arm-set 3 and the seat 5.

An antirotation device, adapted for preventing the rotation of themovable arm-set 3 about the previously mentioned longitudinal axis ofsymmetry, includes a metal bellows 19 fixed to the arm-set 3 and to asuitable mechanical coupling element, or bucket, 20, rigidly coupled(for example glued) to casing 1. The bucket 20 has an axial opening 21and is coupled to casing 1 in such a way to define, with internalsurfaces of casing 1, a recess 22, in communication with the interior ofthe bellows 19.

Two flexible, protective and sealing elements 23 and 24 are arrangedbetween the arm 13 and the casing 1.

The contact occurring between feeler 15 and a piece W is detected, aftercompletion of a determined pre-stroke in the longitudinal direction, or,in the case of displacements of the feeler 15 in the transversaldirection, at a determined angle between the geometric axis of casing 1and the axis of symmetry of the arm-set 3, by means of a detectingdevice including an electric switch, or microswitch 31, rigidly coupledto the casing 1 of the probe, for example, by means of a threadedcoupling 32 and a ring gasket (or “O-ring”) 30.

The microswitch 31, also illustrated in FIGS. 2 and 3, includes ahousing 33 with a longitudinal through hole that defines two cylindricalopenings 34 and 35, with different diameter. A closure plate 37, madefrom insulating material, is fixed to housing 33 by means of rivets orscrews 43 and seals—by compressing in a suitable seat a ring gasket (or“O-ring”) 36—an end of the through hole located at opening 34. Plate 37carries, on its external surface, electrically-conductive tracks 38 and39, shown in simplified, cross-sectional form in FIG. 2.

A cylindrical insulating element 41, for example made from plastic, isinserted in the opening 34 and fitted into it and defines an axial guidehole 42 that is consequently located in a substantially concentricposition with respect to the longitudinal geometric axis.

Stationary contacts are achieved by means of two cylindrical conductivebars 44 and 45, inserted and also fitted into associated mutuallyparallel transversal seats 46 and 47 defined in part by the insulatingelement 41 and in part by internal surfaces of the opening 34, with adisk made from insulating material arranged therebetween, the disk beingrepresented in simplified form by a thick line and identified byreference number 49 in FIG. 2. The disk 49 electrically insulates thebars 44 and 45 from housing 33, the latter being made from conductivematerial.

A small ball 51 made from electrically-conductive material provides amovable contact and is housed, with small clearance, in hole 42. Acompression spring 53, also inserted in hole 42, urges the small ball 51against the bars 44 and 45. When the small ball 51 contacts both bars 44and 45, the microswitch 31 is closed, whereas when the small ball 51disengages from at least one of bars 44 and 45, the microswitch 31 isopen.

Electrically-conductive elements or wires, not shown in the drawings forthe sake of simplicity and clarity, are housed in longitudinal holes 55(one is shown in the figures) and electrically connect the bars 44 and45 to the two conductive tracks 38 and 39, respectively, that are inturn coupled to the leads of a cable, not shown in the drawings, forconnecting—possibly by means of wireless transmission devices, interfacedevices, etc.—the microswitch 31 to a known control and measurementdetecting unit, not shown in the drawing either.

Furthermore, microswitch 31 includes an actuator, or mechanicaltransmission device, 61—for transmitting to the movable contact 51displacements of the movable arm-set 3—substantially housed in thecylindrical opening 35. An elongate mechanical body 63 of thetransmission device 61 is arranged substantially along a longitudinaldirection and includes two abutment ends that protrude from the oppositeends of the opening 35, for cooperating, respectively, with the smallball 51 in opening 34 and with the end of a transmission pin 25, that ispart of the movable arm-set 3 and is substantially aligned along thelongitudinal axis of symmetry.

The elongate mechanical body 63 includes a pushing stem 65 and atransmission element 67 with substantially spherical surface, the latterbeing integrally coupled, for example glued, to an end of the former.

The mechanical transmission device 61 also includes guide elements forthe body 63. More specifically, a bushing 69—per se known, for examplemade from sapphire—is internally fixed to the housing 33, in anintermediate portion with reduced diameter, between the openings 34 and35 and contributes to guide, with limited clearance, longitudinaldisplacements of stem 65 near small ball 51. The internal surface ofopening 35 defines substantially longitudinal guide surfaces thatachieve a binary 71, said surfaces being, in the embodiment shown in thefigures, the lateral ones of a pair of cylindrical bars 70 and 72arranged side by side, glued at the interior of said opening 35. Thecylindrical bars 70 and 72 are made, for instance, from zirconium oxide,which is a material that, like sapphire, does not conduct and hasspecific hardness characteristics, low friction coefficient and highresistance to corrosion and wear.

Among the guide elements there is an elastic thrust element, morespecifically a bent flat spring 73, also housed in opening 35 at anopposite position with respect to the surfaces that define the binary71. More specifically, housing 33 includes, at the part ending withopening 35, a longitudinal slit 75 where the flat spring 73 is insertedin such a way that a larger end 74 of said spring 73 transversallyextends out of the slit 75 (FIG. 3). An annular locking element 76 (forexample, an elastic element) is located in a suitable annular seat ofthe external surface of the housing 33 and prevents the falling of theflat spring 73. The bent flat spring 73 cooperates on one side with thehousing 33 (transversal thrust between the larger end 74 and the wallsof the slit 75) and on the other side with the transmission element 67at a substantially plane portion 77 of its surface—otherwise of aspherical shape—of the latter element.

The flat spring 73 transversally urges the transmission element 67against the surfaces of the binary 71 with an extremely limited force(for example, just a few grams) that is sufficient for causing thelongitudinal displacements of element 67 to be guided by the surfaces ofthe binary 71 free of clearances. Moreover, the specific geometricfeatures and the arrangement of the mechanical body 63 determine that,thanks to the thrust that spring 73 applies to the transmission element67, the pushing stem 65 leans in a repeatable way on an identical areaof the guide surface of the bushing 69, at the opposite side withrespect to the binary 71, so as to achieve that the longitudinaldisplacements of the whole body 63 be guided free of clearance and in arepeatable way, according to a defined kinematic trajectory.

The previously mentioned transmission pin 25 is coupled with a centralpart of the movable arm-set 3 in an adjustable way. More specifically,pin 25 is integral with a threaded dowel 26 that is in turn screwed intoa first threaded end portion of an axial through hole 27 of the arm-set3. The through hole 27 has portions which have different diameter, amongthem a second threaded end portion, for the coupling of the arm 13, andan intermediate threaded area that houses the body of a closure screw28. The head of the closure screw 28 compresses a ring shaped gasket (or“O-ring”) 29 and seals the axial through hole 27.

The part of the through hole 27 facing the interior of the probe, thespace defined by the bellows 19, the recess 22 and the longitudinalthrough hole (openings 34 and 35) of the housing 33 of the microswitch31 define a sealingly closed chamber that is full of inert gas, morespecifically nitrogen (N₂). The gas is inserted in the chamber throughhole 27, prior to sealing by means of screw 28 and O-ring 29.

As previously mentioned, a probe according to the invention can foreseeother embodiments of insofar as the mutual arrangement between movablearm-set 3 and casing 1 in the inoperative condition are concerned. As analternative to the embodiment including some of the characteristicsdisclosed in U.S. Pat. No. 5,299,360 (hereinafter referred to as “ProbeA”) with clearance between flange 11 and casing 1 in the inoperativecondition, there can be foreseen an embodiment in which—in saidinoperative condition—the abutment flange 11 rests on the abutment area7 of the casing 1 (embodiment hereinafter referred to as “Probe B”). Itshould be noted that FIG. 1 wishes to represent both the embodiments(Probes A and B) wherein the clearances of some micrometers—existing inthe first case between the abutment flange 11 and area 7 and in thesecond case between the central portion 9 of the arm-set 3 and the seat5 of the casing 1—cannot be seen.

The operation of the probe according to the two embodiments is asfollows.

When there is no contact between feeler 15 and piece W to be checked,the movable arm-set 3 is supported in casing 1 by the cone-ball typecoupling existing between the portion 9 of the arm-set 3 and the seat 5(Probe A), or between the abutment flange 11 and the abutment area 7(Probe B) and the end portions of mechanical body 63 face the small ball51 and, respectively, the end part of the transmission pin 25. At leastone of the two elements (small ball 51 and pin 25) is separate from theassociated end of body 63 by an extremely limited amount of space, notshown in the figure for the sake of simplicity. The electric circuit towhich the conductive bars 44, 45 and the small ball 51 belong is closed.

Further to mutual displacements between the probe and the piece W, andto contact between the feeler 15 and a surface of the piece W, arm 13and arm-set 3 displace integrally with respect to casing 1.

In the event contact occurs between feeler 14 and piece W along alongitudinal direction (arrow Z in FIG. 1), in the case of both Probe Aand Probe B there occurs a lifting of the movable arm-set 3, and morespecifically of the end surface of the transmission pin 25, with asubstantially translation displacement. Conversely, in the case ofcontact occurring between feeler 15 and piece W along a transversaldirection (arrow X in FIG. 1), in Probe A there generally occurs afirst, limited rotation of the arm-set 3 enabled by the cone-ballcoupling, and the contact that is made substantially at a point betweenthe annular surfaces of the flange 11 and of the area 7 causes a lifting(in other words a displacement including a longitudinal component) ofthe end surface of the pin 25 further to a tilting of the arm-set 3about said point of contact.

In the case of Probe B, contact between feeler 15 and piece W in thetransversal direction X determines, in general, a first, limitedtranslation of the arm-set 3 enabled by the clearance existing betweenthe central portion 9 and the seat 5, and a subsequent displacing awayof the annular surface of the flange 11 from the surface of the area 7,the two surfaces maintaining contact substantially at a point aboutwhich the arm-set 3 tilts. In this case too, the tilting of the arm-set3 causes a lifting (in other words a displacement including alongitudinal component) of the end surface of the pin 25.

In all the briefly described cases, the end surface of the pin 25touches the spherical surface of the transmission element 67 and urgesthe mechanical body 63 that longitudinally translates and is guided freeof clearance by the cooperation between element 67 and binary 71. Thelatter elements are held one against the other by the thrust of thespring 73 that also causes the transversal repeatable leaning of thestem 65 on a portion of the guide surface of bushing 69. Upon completionof a prestroke, the pushing stem 65 urges the small ball 51 to opposethe action of the spring 53 and to open the electric circuit. Theopening of the circuit, that indicates the occurrence of contact betweenfeeler 15 and piece W, is monitored in the control and measurementdetecting unit.

The longitudinal translation displacement of body 63, in addition tobeing guided free of clearance as previously emphazised, is constrainedby the coupling between the flat spring 73 and the substantially planeportion 77 of the transmission element 67. In fact, this coupling limitsor rather prevents, in substance, rotations of body 63 about its axis.Consequently, there is the elimination of, or at least the considerablelimitation of, errors due to the combined action, on the one hand ofunavoidable although minimum misalignments of the component parts of themicroswitch 31 along the longitudinal axis (stem 65, center of the smallball 51 and intermediate point between the bars 44 and 45), and on theother hand of shape, manufacturing or wear defects of the end surfacesof the body 63. Similar shape defects could determine micrometricalterations with respect to the theoretical values of the distancesexisting between the end surfaces of the body 63 and, respectively, thesmall ball 51 and the transmission pin 25. By preventing axial rotationsof the body 63, the values of said distances do not depend on theaforementioned misalignments and shape defects, and remain substantiallyunaltered in the course of the operation of the probe. In this wayduring the probe operation, the distance that the surface of pin 25 hasto cover in order to cause, by urging the elongate body 63, thedetaching of the small ball 51 from the conductive bars 44 and 45 and sodetermine the opening of the circuit is substantially unchanged.

The result of all this is a considerable improvement in therepeatability characteristics of the probe.

As formerly mentioned, the housing 33 of the microswitch 31 is part of asealingly closed chamber filled with nitrogen. This enables to keep theelectrical contacts 44, 45 and 51 in an environment substantially freefrom humidity and oxygen, so avoiding the negative effects due tooxidation and to guarantee a high standard of reliability of the probethat remains constant in time. On the other hand, nitrogen does notinterfere with the metrological performances of the probe, contrarily towhat occurs in known devices in which oily fluids are utilized toprotect the contacts from oxidation.

It should be realized that, in a probe according to the presentinvention, it is possible to substitute nitrogen with a differentsubstance, more specifically an inert gas as, for example, helium (He)or argon (Ar) and achieve identical results.

The insertion and, in general, the handling of an inert gas provide,moreover, considerable advantages with respect to the handling of oilyfluids insofar as the prevention of pollution in the environment isconcerned, since inert gases result definitely “cleaner” and lesstroublesome to handle.

The manufacturing features of the probe, more specifically insofar asthe coupling among the various parts and the selection of the materialsare concerned, can naturally differ with respect to the ones that areillustrated and hereinbefore described without departing from the scopeof the present invention.

Moreover, the structure of the microswitch 31 can take other forms withrespect to what has been hereinbefore illustrated and described. Forexample, there can be foreseen that only one of the bars 44 and 45provides an electric contact, the other being made from non-conductivematerial, while the small ball 51 is coupled to the conductors in theholes 55 for closing the electric circuit in an inoperative conditionand for enabling to detect the opening of the circuit upon the releaseof the engagement between the small ball 51 and the conductive bar (44or 45).

Probes with just some of the advantageous characteristics hereindescribed fall within the scope of this invention. More specifically, aprobe with the microswitch 31 shown in the figures can include, or notinclude, nitrogen or other inert gas in the sealingly closed chamber.

Probes utilizing similar electric-switch detecting devices also fallwithin the scope of the invention even if they have important structuraldifferences with respect to the embodiments of FIG. 1 (Probe A and ProbeB) that can regard, for example, the support of the arm-set 3 in casing1.

1. A touch probe, including a casing that defines a longitudinal geometric axis, a movable arm-set housed in the casing, that defines a longitudinal symmetry axis, an arm rigidly coupled to the movable arm-set, with an end extending out of the casing, a feeler coupled to said end of the arm, and an electric switch adapted for detecting displacements of the movable arm-set with respect to the casing and including a housing, at least one stationary contact, a movable contact, and a mechanical transmission device adapted for transmitting displacements of the movable arm-set to the movable contact, the mechanical transmission device including an elongate mechanical body, located between the movable arm-set and the movable contact, arranged and movable substantially along a longitudinal direction, and guide elements for cooperating with the elongate mechanical body with substantially longitudinal guide surfaces integral with said housing and an elastic thrust element adapted for urging the elongate mechanical body against said guide surfaces.
 2. The probe according to claim 1, wherein the electric switch includes a spring for urging the movable contact against said at least one stationary contact.
 3. The probe according to claim 2, wherein said electric switch includes at least two stationary contacts, said spring being adapted for urging the movable contact against the at least two stationary contacts.
 4. The probe according to claim 1, wherein said substantially longitudinal guide surfaces comprise at least a binary surface, the elongate mechanical body including surfaces adapted for cooperating with said at least a binary surface urged by said elastic thrust element along a transversal direction.
 5. The probe according to claim 4, wherein the elongate mechanical body includes a pushing stem and a transmission element, integrally coupled one with the other, the transmission element defining said surfaces adapted for cooperating with said at least a binary surface.
 6. The probe according to claim 5, wherein the elastic thrust element is arranged between surfaces of said housing and a substantially plane portion of the transmission element, the transmission element defining a substantially spherical surface adapted for cooperating with said at least a binary surface.
 7. The probe according to claim 6, wherein the elastic thrust element includes a bent flat spring.
 8. The probe according to claim 7, wherein the housing of the electric switch includes a longitudinal slit, the bent flat spring being at least partially housed and locked in said longitudinal slit.
 9. The probe according to claim 8, wherein the bent flat spring defines an enlarged end that partially and transversally extends out of the casing through said longitudinal slit, the electric switch including an annular locking element adapted for cooperating with an external surface of the housing for preventing the falling of the bent flat spring from the housing.
 10. The probe according to claim 4, wherein the guide elements include a pair of cylindrical bars, said cylindrical bars define the guide surfaces that comprise said at least a binary surface.
 11. The probe according to claim 1, wherein the movable arm-set, includes a transmission pin, substantially aligned and adjustable along said longitudinal symmetry axis, the transmission pin being adapted for cooperating, further to displacements of the arm, with the mechanical transmission device of the electric switch.
 12. The probe according to claim 11, wherein said substantially longitudinal guide surfaces of the guide elements comprise at least a binary surface, the elongate mechanical body including a pushing stem and a transmission element, integrally coupled one with the other, the transmission element defining surfaces adapted for cooperating with said at least a binary surface, an end of said transmission pin being adapted for contacting, further to displacements of the arm, the transmission element of the elongate mechanical body.
 13. The probe according to claim 1, wherein the movable arm-set is supported in the casing by means of a cone-ball coupling, the movable arm-set and the casing defining annular surfaces adapted for mutually contacting and for causing, further to displacements of the arm, longitudinal displacements of the movable arm-set suitable for being transmitted, by means of said mechanical transmission device, to the movable contact of the electric switch.
 14. The probe according to claim 1, wherein the movable arm-set is supported in the casing by a coupling between plane annular surfaces, the movable arm-set and the casing defining, respectively, a substantially spherical portion and a substantially frusto-conical seat adapted to mutually contact and to cause, further to displacements of the arm, partial disengagement between the plane annular surfaces and consequent longitudinal displacements of the movable arm-set suitable for being transmitted, by means of said mechanical transmission device, to the movable contact of the electric switch.
 15. The probe according to claim 1, wherein the casing encloses a sealingly closed chamber filled with inert gas, the electric switch being arranged in said sealingly closed chamber.
 16. A touch probe, comprising: a casing having a movable arm-set and an arm rigidly coupled to the movable arm-set; a feeler coupled to said arm; an electric switch adapted for detecting displacements of the movable arm-set, said electric switch comprising: a movable contact; and a mechanical transmission device for transmitting displacements of the movable arm-set to the movable contact, the mechanical transmission device including an elongate body between the movable arm-set and the movable contact, and an elastic thrust element for urging the elongate body against a guide surface.
 17. The probe according to claim 16, wherein said guide surface comprises substantially longitudinal guide surfaces of at least a pair of bars.
 18. A touch probe, comprising: a casing having a movable arm-set and an arm rigidly coupled to the movable arm-set; a feeler coupled to said arm; an electric switch adapted for detecting displacements of the movable arm-set, said electric switch comprising: a movable contact; and a mechanical transmission device for transmitting displacements of the movable arm-set to the movable contact, the mechanical transmission device including an elongate body between the movable arm-set and the movable contact, and at least a pair of guide surfaces for cooperating with the elongate body.
 19. The probe according to claim 18, further comprising an elastic thrust element for urging the elongate body against said at least a pair of guide surfaces.
 20. The probe according to claim 18, further comprising a spring for urging said movable contact against at least one stationary contact. 